The present invention relates to a rear wheel steering device for a vehicle.
Conventionally, a four-wheel steering device for a vehicle which steers rear wheels in accordance with a vehicle speed and a steering angle of front wheels has been known in the art. In this device, the rear wheels are steered in the opposite direction at a low speed, and in the same direction at a high speed to the front wheels according to the steered angle thereof.
In this prior art, no problem occurs when turning a corner slowly with a gradual steering of the steering wheel at the initiation of a turn since necessary yaw rate (yaw angular velocity) is attained due to the difference of the steering angles between the front and the rear wheels. However, problems arise when turning the steering wheel abruptly since, with the rear wheels in the same orientation as the front wheels at a high speed, the vehicle moves in a diagonal direction and vehicle yaw rate is restricted. As a result, slip angle .beta. formed between the orientation of the vehicle and the moving direction thereof does not become 0 degrees, which will lead to a failure to meet the driver's desire to turn the vehicle.
DesirabIy, at the initial stage of steering in such a case, first the orientation of the vehicle should be changed, and then the rear wheels should be steered in the same orientation as the front wheel to stabilize the vehicle condition, thereby constantly achieving a slip angle .beta.=0.
To this end, a method of to steering the rear wheels according to the following formula has been proposed: EQU TG.theta..sub.R =-K.sub.F .multidot..theta..sub.F +K.sub.R .multidot.V.multidot..psi. (1)
where
TG.theta..sub.R : target steering angle of rear wheels PA0 .theta..sub.F : steering angle of front wheels PA0 V: vehicle speed PA0 .psi.: yaw rate PA0 K.sub.f, K.sub.R : constants determined by vehicle properties, for example, vehicle weight based on wheels, center of gravity balance of the vehicle. PA0 1.sub.1 : distance between gravity center of vehicle and axle of front wheels PA0 1.sub.2 : distance between gravity center of vehicle and axle of rear wheels. PA0 TG.theta..sub.R : target steering angle of rear wheel PA0 .theta..sub.F : steering angle of front wheel PA0 V: vehicle speed PA0 .psi.: vehicle yaw rate PA0 K.sub.F, K.sub.R : coefficients determined by the properties of vehicle.
In the above formula, K.sub.F and K.sub.R are determined by the following equation: ##EQU1## C.sub.1, C.sub.2 : cornering capacity W: weight
The formula shows that the steering angle of the front wheels functions as an element to steer the rear wheels in the direction opposite to the front wheels while a vehicle speed V and a yaw rate function as an element to steer the rear wheels in the same direction as the front wheels. Accordingly, at a low speed, the direction of the rear wheels become opposite to that of the front wheels since the value of the second term of the formula is small due to the low vehicle speed V. On the contrary, at a high speed, the direction of the rear wheels coincide with that of the front wheels since the value of the second term of the formula is large due to the increase in a vehicle speed V and a yaw rate. However, at the initial portion of the turn, the direction of the rear wheels is opposite to that of the front wheels because the value of the second term of the formula is yet small due to lower yaw rate during the initial stage of the turn.
As disclosed in the U.S. Pat. No. 4,412,594, a device which steers rear wheels in accordance with the output from a yaw rate sensor to compensate for disturbances such as side wind has been known in the art.
In the above formula, K.sub.F and K.sub.R are defined as constants. However, using these values as constants makes it difficult to control the rear wheels. For example, if, during a low speed, the steered angle of the front wheels is approximately at 35 degrees and the rear wheels are steered at the same angles as the front wheels, the vehicle will make a small excessively sharp turn with a resultant swerve of the rear end of the vehicle. Furthermore, drivers accustomed to driving 2-wheel steering cars wherein rear wheels are not steered may feel uncomfortable and uneasy in driving four-wheel steering cars.
According to the above formula, when the wheels are locked, for example, by the abrupt braking, vehicle speed V becomes 0 and the direction of the rear wheels becomes opposite to that of the front wheels. However, since at the initial stage of braking, vehicle speed V is not actually 0, the vehicle will be in an unstable state. In addition, during sudden deceleration, sudden decrease in the vehicle speed V causes the vehicle to move toward the unstable direction or an undesired direction for the driver, with a resultant failure to achieve improved control of the vehicle. Further, during acceleration, the same problem occurs when the wheel spin is occurring.
Above mentioned formula further involves the problem when practically applied to the rear wheel steering device. That is, during sudden acceleration and deceleration, the value of the first term of the formula becomes large, and consequently the element for controlling the rear wheels into the opposite direction to the front wheel becomes large. As a result, the vehicle will be placed in the unstable condition. In detail, when the vehicle speed V becomes 0 due to sudden braking, for example, the value of the second term of the formula becomes 0, and therefore the rear wheels will consistently be steered in the opposite direction. That means the rear wheels are in the direction opposite to the front wheels when the control of the vehicle is recovered in the driver's hands, which is not desirable transiently. Furthermore, even during the low speed running at the initial stage of the acceleration, the value of the first term of the formula should be small to achieve the stability of the vehicle condition since the driver does not require the turn of the wheels at that stage.
On the other hand, at a low speed, the value of the second term of the formula should be small. This is because when driving at a low speed, for example, while garaging a car, vehicle speed fluctuates substantially due to frequent change in the degree of acceleration, and relatively high vehicle yaw rate is generated due to the large steering angle. Under such circumstances, extent of degrees to which the rear wheels are steered opposite to the front wheels decreases to the contrary of the driver's desire to make a small turn. In addition, the driver may feel uncomfortable because of the frequent changes of the steering angle.
When practically applied to the rear wheel steering device of a vehicle, the method of controlling the rear wheels based on a vehicle yaw rate has an advantage over the conventional two-wheel steering device under a certain condition. That is the stable controllability of the vehicle under the condition that the change of speed of the vehicle yaw rate is within the fixed range while the rear wheels are securely steerable. However, the disadvantage lies in the difficulty of recovering the control of the rear wheels, once it is lost. For instance, according to the result of the test conducted to determine when the control of rear wheels is lost at the time of exceeding the peak of the cornering power C.sub.P (refer to FIG. 17) during the turn under the same road condition, four-wheel steering vehicle lost its control at a higher speed while two-wheel steering vehicle lost its control at relatively low speed. This indicates that the four-wheel steering vehicle lose its controllability while the kinetic energy of the vehicle is high, resulting in the subsequent difficulty of recovering the control. Accordingly, slip angle .beta.=0 cannot be obtained.